The invention relates generally to gas dryers, and more particularly to a variable evaporator control (“VEC”) system and method for a refrigerated compressed gas dryer which provides for varying the refrigerant temperature responsive to changes in the compressed gas load on the refrigerant compressor.
Refrigerated compressed gas dryers are used to remove moisture and water vapor from compressed gas streams which are introduced through the gas compressor intake when the compressed gas is taken from the ambient with its accompanying relative humidity. Once the gas is compressed, its vapor holding capacity is reduced and the vapor condenses into a liquid as the gas is cooled. Prior art type refrigerated compressed gas dryers basically consist of a pre-cooler/re-heater heat exchanger, an evaporator heat exchanger, a liquid separator and a liquid drain valve. The warm compressed gas is passed through the pre-cooler/re-heater where it is cooled by the outgoing cooled gas stream. As the warm compressed gas stream is cooled moisture vapor begins to condense into a liquid. The compressed gas stream is then passed into the evaporator heat exchanger where it is further cooled to a lower temperature as the refrigerant evaporates at some temperature below the desired temperature of the compressed gas stream exiting the evaporator. More water vapor is condensed into a liquid state in the evaporator and the cooled gas stream is passed to the liquid separator where the condensed liquid is separated and removed from the system by the drain valve. The cooled and dried compressed gas stream is then returned through the pre-cooler/re-heater to pre-cool the warm incoming compressed gas stream prior to being returned to the compressed gas system piping. Compressed gas flow rates will vary as a function of time in nearly every compressed gas dryer application. The equipment can be expected to experience flows ranging from the maximum design flow rate down to a no-load, or zero, flow rate condition.
The refrigeration system of a typical refrigerated compressed gas dryer as described above basically consists of a refrigerant compressor, a refrigerant condenser, an expansion/restrictive device, and the evaporator described above. The temperature of the cooled compressed gas, as it exits the evaporator, defines the thermal performance of compressed gas dryers. This is typically expressed at the design flow rate. Increased cooling of the warm compressed gas results in lower exiting evaporator compressed gas temperatures and higher levels of moisture removal. However, there is a practical limit to the amount of cooling that can be done in the evaporator of a refrigerated gas dryer. Cooling the warm compressed gas stream down to a temperature below the freezing point of water creates a situation where the condensate can freeze and block the free path of the compressed gas stream, thus, increasing the pressure drop across the dryer. In extreme circumstances, the flow can be blocked completely, starving the downstream process of compressed gas. This failure situation will most likely occur during compressed gas flow rates that are much less than the maximum design flow rate. When using evaporators constructed from smooth tubing, the freeze-up failure potential necessitates that the refrigerant temperature in the evaporator be above the freezing point of water, and held fixed and steady, as the load varies from no load to full load. All manufacturers of refrigerated compressed gas drying equipment must address how to control the evaporator refrigerant temperature in order to prevent condensate freeze-up under low or no load operating conditions, while providing the thermal performance advertised at a full load situation.
Presently, the most common method of controlling the evaporator refrigerant temperature in the compressed gas dryer is through the use of a hot gas by-pass valve, which is a pressure-regulating valve that is set to maintain a constant refrigerant pressure in the evaporator and refrigerant compressor suction line. The by-pass valve operates by metering high-pressure refrigerant discharge gas into the refrigerant compressor suction line whenever the suction pressure drops below the set point of the pressure regulating by-pass valve. By understanding the saturation temperature/pressure correlation of the refrigerant gas, the evaporator refrigerant temperature can be indirectly regulated by maintaining a constant refrigerant suction pressure. This temperature/pressure correlation refers to the unique physical saturation properties of each refrigerant; that is, as a refrigerant changes phase from a liquid to a vapor (i.e., boils or evaporates), it will do so at a constant temperature and pressure. If the pressure is controlled and maintained while this phase change occurs, the temperature is also maintained. Therefore, the more precisely the pressure is maintained, the more accurately the evaporator temperature: is held constant. A typical pressure setting for the by-pass valve would be a refrigerant saturation pressure that corresponds to a saturation temperature of approximately 35 degrees Fahrenheit. Placing the equivalent temperature setting slightly above the freezing point of water allows for a small factor of safety in the event of any valve setting drift.
Another commonly used method to maintain a constant refrigerant suction pressure is to install an automatic pressure valve (“APV”) in place of the expansion/restrictive device and the hot gas by-pass valve. The APV maintains proper refrigerant suction pressure by metering high-pressure liquid refrigerant into the inlet of the evaporator. The APV is typically inexpensive and inaccurate. Under no-load conditions, the liquid refrigerant may not be effectively converted into a gas in the evaporator, which can result in a liquid flood-back condition at the refrigerant compressor suction, with potential compressor damage. Also, as the load is applied to the dryer, the refrigerant suction pressure often increases, resulting in poor thermal performance. Some of the newer technologies used to maintain a constant refrigerant suction pressure include the use of variable speed refrigerant compressors which operate by altering the rotational speed, and therefore, the pumping capacity of the compressor. The refrigerant suction pressure can be increased or decreased by decreasing or increasing, respectively, the rotational speed of the compressor. Regardless of the manner of controlling the suction pressure, typical prior art control schemes function to maintain a constant suction pressure, and thus a constant evaporator refrigerant temperature, regardless of the load on the compressor. Consequently, prior art methods can suffer problems such as lower efficiency or freeze up conditions during compressor no-load conditions.
Many conventional compressed gas dryers utilize smooth tubes in the evaporator, which offer the advantage of a non-fouling surface that performs consistently throughout the life of the dryer. Other advantages are reduced pressure drop and relatively inexpensive manufacturing costs. A disadvantage of smooth tube technology is that a relatively large amount of heat exchange surface is necessary in order to achieve the desired thermal performance at the design full load condition. This can be particularly challenging when considering the no-load and partial load freeze up concerns discussed previously, as well as the need to operate the evaporator at 35 degrees Fahrenheit, offering a 4 degree Fahrenheit approach temperature. The efficient packaging of these dryers can be inherently more difficult. Extended surface heat exchanger tubes are often used in order to make the evaporator more compact. The externally finned surface of such designs offer a temperature gradient between the refrigerant and the compressed gas stream. This gradient can permit the refrigerant temperature to be less than the freezing point of water, without the danger of freeze-up. A reduced refrigerant temperature results in a larger temperature approach, and less required surface area. While the length required for this design is reduced as compared to the smooth tube designs, the cost of the tube, and the design, can generally be greater. The designer may also have to address the concerns of excessive pressure drop.
Small, compact heat exchangers, such as brazed plate, or bar and frame type heat exchangers, offer an extremely attractive packaging solution for a compressed gas dryer, but, again, can be much more costly than the smooth tube designs. As these designs do not incorporate extended surfaces and the above discussed temperature gradients, the refrigerant temperatures must remain above the freezing point of water in order to perform reliably under all operating conditions. A precise and constant evaporator refrigerant temperature control is imperative to these designs.
Due to the factors explained above, there has generally been no single optimum heat exchanger design for a compressed gas dryer. A problem has been that prior art designs are configured to maintain a constant suction pressure, and thus evaporator refrigerant temperature, regardless of the compressed gas load on the refrigerant compressor. Consequently, there has been a compromise between the desired features of thermal performance, pressure drop performance, reliable operation, size, cost and packaging. The shortcomings of prior art refrigerated compressed gas systems described above illustrates the need for a control system for a refrigerated compressed gas dryer which can vary the evaporator refrigerant temperature in response to changes in the load on the refrigerant compressor. Consequently, the cooling capability, per-unit length, of any given length heat exchanger can be maximized.